Friday, March 25, 2011

Repairing the "konged" Honda CB750

Last sunday the SG crew came over and we did some repair work on my '74 CB750 bobber.
Over the past decade of abusing her across 14 states and thousands of miles, the rails behind the seat area had bent forward. CB750 tubing is quite thin walled and the area wasn't gusseted initially, mainly because of how the seat fits on. I guess it could only take so much pounding from the rear before it finally just had to give.

We started by firing the torch and bending the rails back to their straight and level postion. When I initially noticed that the rails were bent, I had suspected there were cracks. Once heated and bent back, the areas around the welds opened right up. This led me to believe the frame had actually been cracked as I had thought.

Once we determined that simply bending them back into position was a pipe dream, we opted for what I had planned on doing from the beginning, cut and replace. We made sure the bike was sitting level and measured up from the lower engine mount area 7.5" to where we would cut off the bent rails. After double checking our measurements a few times and marking them with tape, we cut em off.

Next we cut two 4.5" pieces of tubing that had a slightly smaller o.d. than the i.d. of the frame's tubing for sleeves. This would allow 2.25" on each side of the seam. We then measued up the same distance on a donor stock rear end (Thanks Dock!!! http://vintagepureheaven.blogspot.com/) and cut them in the same place we cut off the junk ones. We then had our two new replacement pieces for the seat/fender rails.

After cleaning up the metal and drilling holes for plug welds we thought we'd see how it all fit together. The left side slid right on and with a few light raps from a rubber mallet seated itself nicely. The right side was a bit more stubborn as there was a slight burr about 1/2" from our cut inside the frames tubing. After realizing that trying to file it down wasn't gonna work out, we opted for sliding the rail down on the sleeve as far as it would go and applying some heat to it while tapping it with a BFH until that burr was soft enough that it slid down to where it needed to seat.

We checked the levelness periodically thru this process. We also checked for centeredness before final welds. Once we had it all in place it was tacked and welded.

I mentioned above the lack of room for gussets. Even though the tubing was now twice as thick in the problem area, it would still be susceptible to stress because of shock angle and where its located on the frame. We put the fender and seat on to determine how much space we actually did have to work with. We figured if we angled the gussets that we could squeeze them in between the fender and the bottom of the seat pan.

Some cardboard patterns were then sketched and then gussets were cut from 3/8" steel...nice and beefy. Where the gussets are located will be completely hidden from sight once the fender and seat are reinstalled. The extra metal that you see in some of these pics was also hacked off, the old fender brace/mount and the left over portion of the stock seat rails.

I still have to clean up the metal a bit and get some paint on 'er. I'm also going to do tend to some other issues and things I've been meaning to get to. With her this far torn down, its time to stop the abuse and apply a little TLC.